mei: bus: move driver api functions at the start of the file

To make the file more organize move mei client driver api
to the start of the file and add Kdoc.

There are no functional changes in this patch.
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>